Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Chemical compound
Cyclodecapentaene or [10]annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry.[1]: 121–122
Conformation, strain, and non-aromaticity
1): a hypothetical planar conformation for all-cis-[10]annulene.
(2): the lowest-energy conformation for all-cis-[10]annulene
(3): one hypothetical planar conformation for trans,cis,trans,cis,cis-[10]annulene
(4): one conformation for trans,cis,cis,cis,cis-[10]annulene
Although not aromatic itself, [10]annulene can transition between different conformational isomers through aromatic or quasiaromatic excited states, such that its conformational isomerism is fixed only at extreme cryogenic temperatures.[2] Understanding the composition and reactivity of these mixtures computationally has proven difficult,[3] because a large number of conformations all minimize the energy locally.[4]
The all-cis isomer (1), a fully convex decagon, would have bond angles of 144°, which creates large amounts of angle strain relative to the ideal 120° in sp2 atomic hybridization. Instead, the all-cis isomer adopts a planar boat-like conformation (2) to relieve the angle strain,[5] although it, too, is less stable than the next planar isomer, trans,cis,trans,cis,cis-[10]annulene (3).[citation needed] Yet even isomer (3) is unstable, suffering from steric repulsion between the two internal hydrogen atoms,[6] and tends to distort into the perimeter of two fused circles, one larger and the other smaller, as in azulene.[2] The nonplanar trans,cis,cis,cis,cis isomer is the most stable of all possible isomers,[citation needed] although it is unclear whether it too has a boat-like configuration as in conformer (4), or the "heart" configuration produced if one internal hydrogen in conformer (3) were flipped inside-out.[2]
Synthesis
Cyclodecapentaene can undergo an electrocyclic rearrangement to[7] or from dihydronaphthalene. Photolysis of the latter generates [10]annulene, but it quickly reverts to the reactant, even at cryogenic temperatures.[1]: 122
Aromatic derivatives
(5): an aromatic bridged [10]annulene(6): An aromatic tricyclic [10]annulene
Aromaticity can be induced in compounds having a [10]annulene-type core if planarity is forcibly imposed by other substituents. Two methods to do so are known.
When deprotonated to form the anion this type of compound is even more stabilized. The central carbanion enhances the molecule's planarity and the number of resonance structures that can be drawn is extended to 7 included two resonance forms with a complete benzene ring. Computational chemistry suggests a tricyclic[10]annulene derivative with an annulated benzene ring and a full set of cyanosubstituents (7) would be one of the most acidic compounds known, with a computed pKa in DMSO of −30.4 (compared to for instance −20 for magic acid).[9]
The other method is to further remove hydrogens and develop triple bonds or cyclopropanes along the ring. Thus computational studies suggest that cyclodecatetraeneyne is (although formally a 12-π system) planar and aromatic,[10] as is bicyclo[8.1.0]undeca-1,3,7,9-tetraen-5-yne.[11] Predicting the aromaticity of these compounds is not always obvious: the polycyclic hydrocarbon tetradihydronaphtho[10]annulene, in which a valence isomer of [10]annulene is fused to two naphthalenes, does not exhibit aromaticity inside the central 10-π ring.[12]
Other related compounds
Azulene is also a 10 π-electron system in which aromaticity is maintained by direct transannular bonding to form a fused 7–5 bicyclic molecule.
Cyclodecatetraene is a stable, non-aromatic 8 π-electron system with no ring strain.[1]: 131
References
^ abcKemp-Jones, A. V.; Masamune S. (1973). "The monocyclic 10π-electron system". In Nozoe Tetsuo; Breslow, Ronald; Hafner, Klaus; Itô Shô; Murata Ichiro (eds.). Topics in Nonbenzenoid Aromatic Chemistry. Vol. I. Tokyo/New York: Hirokawa Publishing, on behalf of Halsted Press, a division of John Wiley & Sons. pp. 121–131. ISBN 0-470-65155-5 – via the Internet Archive.
^ abcCastro, Claire; Karney, William L.; McShane, Colleen M.; Pemberton, Ryan P. (2006-04-01). "[10]Annulene: Bond Shifting and Conformational Mechanisms for Automerization". The Journal of Organic Chemistry. 71 (8): 3001–3006. doi:10.1021/jo0521450. ISSN 0022-3263.
^ abSlayden, Suzanne W.; Liebman, Joel F. (2001-05-01). "The Energetics of Aromatic Hydrocarbons: An Experimental Thermochemical Perspective". Chemical Reviews. 101 (5): 1545–1546. doi:10.1021/cr990324+. ISSN 0009-2665.
^Xie Yaoming; Schaefer, Henry F.; Liang Guyan; Bowen, J. Phillip (Feb 1994). "[10]Annulene: The Wealth of Energetically Low-Lying Structural Isomers of the Same (CH)10 Connectivity". Journal of the American Chemical Society. 116 (4): 1442–1449. doi:10.1021/ja00083a032. ISSN 0002-7863.
^Xie et al. 1994, though note that Kemp-Jones & Masamune 1973, pp. 126–7 instead proposes a "twist" conformation, with 6 atoms coplanar and the remaining 4 in a raised handle.
^Sulzbach, Horst M.; Schleyer, Paul v. R.; Jiao Haijun; Xie Yaoming; Schaefer, Henry F. (Feb 1995). "A [10]Annulene Isomer May Be Aromatic, After All!". Journal of the American Chemical Society. 117 (4): 1369–1373. doi:10.1021/ja00109a021. ISSN 0002-7863.
^Masamune S.; Seidner, R. T. (1969). "[10]Annulenes". Journal of the Chemical Society D: Chemical Communications (10): 542. doi:10.1039/c29690000542. ISSN 0577-6171.
^Gilchrist, Thomas L.; Rees, Charles W.; Tuddenham, David; Williams, David J. (1980). "7b-Methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylic acid, a new 10-electron aromatic system; X-ray crystal structure". Journal of the Chemical Society, Chemical Communications (15): 691–692. doi:10.1039/C39800000691.
^Vianello, Robert; Maksi, Zvonimir B. (2005). "Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO—a density functional study". Chemical Communications (27): 3412–3414. doi:10.1039/B502006A. PMID 15997281.
^Navarro Vázquez, Armando; Schreiner, Peter R. (2005-06-01). "1,2-Didehydro[10]annulenes: Structures, Aromaticity, and Cyclizations". Journal of the American Chemical Society. 127 (22): 8150–8159. doi:10.1021/ja0507968. ISSN 0002-7863.
^Parmar, Karnjit; Blaquiere, Christa S.; Lukan, Brianna E.; Gengler, Sydnie N.; Gravel, Michel (Sep 2022). "Synthesis of a highly aromatic and planar dehydro[10]annulene derivative". Nature Synthesis. 1 (9): 696–700. doi:10.1038/s44160-022-00135-z. ISSN 2731-0582.
^Umeda Rui; Hibi Daijiro; Miki Koji; Tobe Yoshito (2009-09-17). "Tetradehydrodinaphtho[10]annulene: A Hitherto Unknown Dehydroannulene and a Viable Precursor to Stable Zethrene Derivatives". Organic Letters. 11 (18): 4104–4106. doi:10.1021/ol9015942. ISSN 1523-7060.
